Funds are requested in partial support of the third Conference on Folic Acid, Vitamin B12 and One-carbon Metabolism to be held under the auspices of the Federation of American Societies for Experimental Biology (FASEB) from July 29 to August 3, 1990 at Vermont Academy, Saxton's River, VT. The prior two meetings on this topic were held in 1986 (at Saxtons River, Vermont) and in 1988 (at Copper Mountain, Colorado), and were very well received by the 75 participants at the first and 140 at the second conference, including leading scientists and bright young scientists on their way up a variety of disciplines and from both basic science and clinical backgrounds. Discussion leaders nationally recognized for their insights and productivity have been selected for the nine sessions in the 1990 Conference, all agreed and obtained the consent of outstanding speakers to give presentations in their respective sessions. The detailed tentative program is presented. Poster sessions will also be held. Major foci of the meeting will be recent advances in the understanding of DHFR and other enzymes with folate (and folate polyglutamate) or cobalamin substrates which can become targets for therapy, recent significant studies of one-carbon metabolism in lower organisms, the ways in which such information may indicate new therapies of protozoal and other diseases, with particular focus on those organisms which are major opportunistic infecting agents in AIDS patients. The latest information will be presented on inherited disorders, neurobiochemical abnormalities of folate, cobalamin and methionine, the effects of drugs on folate and cobalamin metabolism, and the machinery of transport, delivery, and export of these vitamins and their derivatives. Participation will be limited to 150 and applicants will be selected on the basis of their expertise and interests as most likely to contribute to a stimulating meeting which will increase the insights of all who attend, leading to more integrated research in the Conference topics and more effective therapy of a number of major infective, hematologic, neoplastic, and neurologic disorders.